The FGFR (fibroblast growth factor receptor) family of receptor tyrosine kinases is important for cell growth, differentiation, migration, wound healing and angiogenesis. This revised proposal will examine the role of aberrant FGFR3 signaling in the recruitment and activation of effector proteins that mediate proliferative and antiapoptotic pathways. Activating mutations have been identified in FGFR3 which contribute to the progression of multiple myeloma and certain carcinomas. Many important features of FGFR3 signaling remain unclear, and will be examined by the experiments described in this proposal. Since the last submission of this grant, we have made significant progress towards understanding signaling pathways downstream of activated FGFR3. First, we have further characterized the interaction of FGFR3 and the adapter protein SH2-Bbeta, demonstrating an important role for SH2-Bbeta in the activation of Stat5B. Second, our preliminary data identify Pyk2/RAFTK as an interacting protein with FGFR3. Current data in the literature document the role of Pyk2, and the associated phosphatase Shp2, in controlling apoptosis in multiple myeloma cells. Thus, its interaction with FGFR3 elevates Pyk2/RAFTK to a position of paramount importance as an effector of FGFR3 signaling. Aim 1 examines four proteins which potentially interact with FGFR3, identified in a two-hybrid screen using a weakly activated form of FGFR3 as the bait. These proteins are: SH2-Bbeta, p8515, FRS2alpha and PLC-gamma. We will analyze the specific protein domains and potential phosphorylation sites of SH2-BI3, p85beta, FRS2alpha and PLC-gamma that are required for interaction with FGFR3. Aim 2 examines the newly discovered interaction between Pyk2/RAFTK and FGFR3, subsequent recruitment of Grb2 and src, leading to MAPK activation. In addition, this aim examines effector proteins associated with PI 3-kinase, such as Akt and PLC-gamma, that are recruited to FGFR3 signaling complexes. In addition, PI 3-kinase can contribute to mitogenic signaling in cells through activation of PLC-gamma. Thus, we will examine the role of PLC-gamma and PI 3-kinase in FGFR3-induced cell motility. Aim 3 will examine the mechanism of Stat activation that accompanies signaling by many RTKs, including FGFR3. In this aim, we wish to examine the mechanism of Stat recruitment and phosphorylation by activated FGFR3 receptors. This aim is strengthened by our recent published results demonstrating a role for SH2-Bbeta in the activation of Stat5B by activated FGFR3. This aim will also examine whether FGFR3-mediated Stat activation and phosphorylation requires the recruitment of JAKs.